In orthogonal frequency division multiplexing (OFDM), the transmission band is divided into multiple orthogonal carrier waves. Each carrier wave is modulated according to the digital data to be transmitted. Because OFDM divides the transmission band into multiple carriers, the bandwidth per carrier decreases and the modulation time per carrier increases. Since the multiple carriers are transmitted in parallel, the transmission rate for the digital data, or symbols, on any given carrier is lower than when a single carrier is used.
OFDM modulation requires the performance of an Inverse Fast Fourier Transform (IFFT) on the information to be transmitted. A Fast Fourier Transform (FFT) is used for demodulation. In practice, the IFFT and FFT are provided by digital signal processing carrying out an Inverse Discrete Fourier Transform (IDFT) and a Discrete Fourier Transform (DFT), respectively. As such, the characterizing feature of OFDM modulation is that orthogonal carrier waves are generated for multiple bands in a simultaneous fashion within a transmission channel.
One of the primary benefits of using OFDM modulation for communications is to enhance the robustness of the system against selective frequency fading and narrow band interference. Unlike single carrier systems, fading in a certain frequency range or interference in a certain frequency range will not cause the entire communication link to fail. In an OFDM system, only a small percentage of the parallel carriers will be affected by fading or interference at a given frequency.
Recently, the IEEE has adopted OFDM for certain 802.11 and 802.16 communication standards. These standards provide solutions for high data rate transmission in broadband systems. Although data is the current focus in these systems, voice services will need support in future systems. Unfortunately, OFDM presents certain obstacles for voice services. In particular, OFDM systems have large numbers of carriers that must be assigned and allocated for different services and users. As channel conditions vary, carrier allocation must vary to ensure efficient use of resources, while maintaining a desired quality of service.
Accordingly, there is a need to control allocation and assignment of OFDM carriers to accommodate voice traffic, while optimizing system resources and maintaining a desired quality of service. In particular, there is a need to ensure desired transmission rates while minimizing the number of carriers allocated to a given user.